This invention relates to a stencil printing process for circuits. More particularly, this invention relates to a system for determining the proper setting for a plurality of parameters for stencil printing equipment in order to optimize the placement, accuracy, and volume of solder paste on a circuit and reduce the number of defects.
In today""s society, most electronic equipment is made of one or more circuit boards. Circuit boards are a plastic or other non-conductive material having leads placed on the material which connect two or more integrated circuits chips affixed to the board. A stencil printing machine places solder paste on the board in a pattern that was pre-programmed in the machine. It is a problem that the placement of the solder paste onto the circuit board must be precise in both placement and height. If the placement of solder paste is wrong, the alignment between a pin from a chip or other circuit and the lead on the board may have a gap causing a short in the circuit. If the height of the solder paste on the board is inconsistent from lead to lead, a pin from a chip or another circuit may either not contact the lead or may protrude too far into the lead. These may cause a short or other problems in the circuit.
As circuits become smaller, the space between leads on a board decreases. As the space between leads decreases, the placement and height of the solder paste becomes more critical. If the placement of a lead is wrong, it may become to close to an adjacent lead which may cause a short or other problem. If the height is too great, the leads may cause an arc between leads.
For the above reasons, placement of the solder paste on a board is critical. The placement of the solder paste is controlled by the following parameters: squeegee speed of the printing machine, angle of the squeegee, time of separation, snap-off distance, head pressure, and squeegee material. Squeegee speed is the speed that the printer head moves across the board. Angle of the squeegee is the angle of the squeegee blade with respect to the board. Time of separation is the amount of time that the printer allows the stencil to remain on the board in order for the solder paste to settle in position on the board before the stencil is removed. Snap-off distance is the distance from the top of the board to the bottom of the stencil. Head pressure is the amount of force applied by the squeegee to the paste applied to the board. Squeegee material can be metal or rubber. In the past, experimentation as well as trial and error were used to set these parameters. However, both of these methods are imprecise and time consuming to accomplish. There is a need in the art for a system to determine the optimal settings of these parameters.
The above and other problems are solved and an advance in the art is made by the provision of the system for determining optimal settings of a stencil printer in accordance with the present invention. The present invention reduces the time needed to determine optimal settings for the printer by providing an automated system that does not rely upon experimentation as well as trial and error. Furthermore, the system of the present invention can monitor the current settings and results to modify the settings and increase the accuracy of the settings.
The system of the present invention is provided by instructions executed by a either a controller attached to a stencil printing machine or by a computer system that is not connected to the operation of the stencil printing machine. The controller adjusts the settings of the stencil printing machine and the computer outputs the correct settings for the machine which then must be manually entered into a stencil printing machine. The instructions executed by either the controller or a computer are stored in a machine readable format on a memory connected to a processor.
The process for optimizing the settings of a stencil printing machine embodied in the instructions is performed in the following manner. First, the system collects data relating the settings of various parameter of the stencil printing machine to the resulting output parameters. The input parameters include squeegee travel speed, angle of the squeegee, time of separation, snap-off distance, head pressure, and squeegee material. The output parameters include but are not limited to settings for the above parameters, print deposit thickness, and print deposit width.
The data is used to generate a neural network that relates the input parameters to the output parameters. A neural network develops correlations between system inputs and outputs that may not have a linear relationship. The correlations are developed by a series of weighted expressions that relate the input to the output. After the neural network is generated, back propagation is used to adjust the input values to optimize the settings of the stencil printing machine for an optimal result. The adjusted input values are then output by the process. The adjusted values then may either be directly applied to the stencil printing machine or a user must adjust the settings manually. Additionally, the system of this invention may also monitor the procedure as the stencil machine operates to receive data that can then be periodically used to calibrate the neural network to make correlations between input parameter and output parameters more precise.